The present invention relates to a slip-controlled hydraulic brake system for all-wheel driven motor vehicles equipped with lockable differentials. The system comprises a variety of sensors for measuring the rotational behavior of the wheels, and electronic circuits for logic combining and processing the sensor signals and for generating brake pressure control signals by way of which the brake pressure in the individual wheel brakes is variable in response to the rotational behavior of the wheels and to a vehicle speed reference variable.
In motor vehicles with all-wheel drive and slip-controlled brake systems, the transmission of drive and brake torques, by way of the drive shafts, and the elevated moments of inertia, envolves difficulties in measuring and interpreting the rotational behavior of the wheels and the regulation or modulation of the brake pressure derived thereform. By differential locks, the transmission of the brake torques from one wheel to the other wheel is further increased, thereby adding to the difficulties of the brake slip control. To overcome these difficulties, it is basically conventional practice to de-activate the slip control when engaging the differential locks. However, this envolves the disadvantage that in the event of unfavorable road or wheather conditions in which differential locks are required, one must forego the slip control, although the slip control under these conditions could accentuate its advantages and preserve the steerability and the driving stability of the motor vehicle.
According to another suggestion for overcoming this problem, the differential locks are automatically released upon commencement of the brake slip control (German Patent Application P No. 3418520.8 printed and published Nov. 21, 1985. However, this will require switchable differential locks, for which reason this solution is unsuitable, for example, for motor vehicles containing viscose couplings as differential locks.
Moreover, it has already been suggested to mount, in at least one of the split axles transferring the driving torque from a differential to a wheel, a disconnecting clutch automatically disengageable by the brake slip control signal (German Patent Application P No. 3426747.6) printed and published Jan. 30, 1986. However, such additional mechanical efforts are not always acceptable.
It is, therefore, the object of the present invention to overcome these shortcomings of these brake systems and to develop a brake system suitable for all-wheel driven motor vehicles which also permits a brake slip control with locked rear axle and/or distibution differentials, thereby safeguarding in any situation the steerability and the driving stability of the motor vehicle by preserving a short stopping distance.